Friction pumps produce, in the range of molecular flow, a constant pressure ratio and, in the range of laminar flow, a constant pressure difference. In friction pumps of the Gaede, Holweck or Siegbahn, construction type, for instance the pressure ratio in the molecular range as well as the pressure difference in the laminar range are especially high at very narrow gaps. Turbomolecular pumps, being a refinement of friction pumps of the earlier type of construction, produce a very high pressure ratio in the molecular range even at larger gaps. In the laminar range, however, they produce only a very small pressure difference.
A friction pump constructed according to Holweck is, for instance, described in Comptes rendus Acad. Science 177 (1923) 43 and following. A friction pump of the type built by Siegbahn is described in the Archives of Mathematics and Astrology and Physics 30 B (1943). The basic construction and the mode of operation of a turbomolecular pump according to Becker are described in Vacuum Technology 9/10 (1966).
The operational range of turbomolecular pumps is limited towards higher pressures, since they are only fully effective in the molecular flow region. This molecular flow region is bounded by that pressure, for which the average free travel length of the molecules comes down to the order of magnitude of the vessel dimensions. Therefore, turbomolecular pumps operation only in combination with pre- or fore-vacuum pumps, generally two stage rotary vane is now pumps. If, however, it can be achieved to extend the operating range of turbomolecular pumps towards higher pressures, the effort to produce the necessary fore-vacuum can be reduced. In that case, for instance, single stage rotary vane pumps, and after would be sufficient, or, the oil tight rotary vane pumps could be replaced by dry diaphragm type pumps.
It is possible to extend the working range of a turbomolecular pump towards higher pressures by placing a friction pump of the Holweck or Siegbahn construction type downstream on the pre-vacuum stage. Such combinations are described, for instance, in the DE-AS 24 09 857 and in the EP 01 29 709.
For the operation of such frictional pumps, it is essential that the spacing between rotor and stator is very small. Only then they still operate in the molecular flow range even at higher pressures than a conventional turbomolecular pump and they develop their entire pressure ratio. Thus, the working range is extended towards higher pressures.
A high rotational speed of the rotor is another premise for an effective mode of operation of a friction pump. Herein, the rotor bearing type support has great importance. Apart from the classical mechanical rotor support by lubricated ball bearings, combinations of permanent-magnetic bearings and ball bearings are used today.
For completely contact-free support or operation, actively controlled magnetic bearings of various types can be used. These require, however, in addition to the magnetic bearing elements, a mechanical emergency bearing arrangement. If a magnetic bearing or parts thereof fail during operation, the emergency bearings have the task to support the rotor. This can occur in the course of a short malfunction, after which the rotor is again being levitated by the magnetic bearings, or also after a longer malfunction, wherein, the rotor is carried by the emergency bearings until the coasting stage is reached. In both cases, the emergency bearings must be designed, in such a way that a safe rotor operation is assured without contact with the very closely adjacent stator disks.
It follows herefrom that the gaps between rotor and emergency bearings must be significantly smaller than the anyway very narrow gaps in the region of the pump elements. At higher rotational speeds, an unbalance (technical term) leads to movements of the rotor which furthermore limit the clearance in the emergency bearing.
It is advantageous to utilize control methods which result in an operation of the rotor without any forces acting upon it. Herein, the movements of the rotor caused by unbalance are not controlled in order to avoid transmitting the corresponding reaction forces to the housing. During operation, the rotor unbalance can, however, increase due to different reasons, which also increases the amplitude of the rotor movement. This can only be tolerated in a very limited range so that no contact with the stator parts can occur.
These extreme requirements, high rotational speed, minimum gaps between the stationary and rotating parts of the pump elements and the emergency bearings as well as the necessity of a safe operation, also in case of a malfunction of the magnetic bearing, build up conditions for the designer of such a frictional pump, which can only be made compatible with the greatest difficulties.
It is possible to approach a solution of these problems by a high accuracy machining. This means, however, a large expenditure of work and technical equipment. Sensor technology also provides solutions, which, as well, are characterized by a effort in work technology and financial expense.